I would absolutely pay for collapsible comment threads. I hate comment systems that don't allow it, and it's one of the reasons I love Disqus. While we're requesting features, can HN add this to the site?
You can use HNES (https://github.com/etcet/HNES) for Chrome to get collapsible comments. You can also use my fork (https://github.com/ibejoeb/HNES) to try some enhancements to the collapsible comments, which does persistent collapses and new comment indicators on the parent thread.
